1. Field of the Invention
The present invention relates to a processing method and a processing apparatus for removing by a wet etching a stacked films formed of at least two films which comprise a first film and a second film of a target object to be processed such as a semiconductor wafer, and a computer program, and a computer readable storage medium.
2. Description of the Related Art
In recent years, the design rule of the semiconductor elements constituting an LSI is being made finer and finer in compliance with the demands for further improvements in the degree of integration and the operating speed of the LSI. In this connection, the gate insulating film is required to be made thinner in the MOS device.
Conventionally, a silicon oxide film or a silicon oxynitride film has been used for forming the gate insulating film. In order to obtain a desired performance by using the gate insulating film formed of the material noted above, it is necessary to form the gate insulating film very thin. However, if the gate insulating film is made thinner, the gate leak current is increased, thereby increasing the operating power.
Under the circumstances, a so-called “high-k material” such as HfO2, which has a relative dielectric constant higher than that of the conventional material, attracts attentions. Such being the situation, a gate insulating film of a two-layer structure prepared by forming a high-k material film on an interfacial oxide film consisting of a very thin SiO2 film has come to be used in place of the conventional gate insulating film formed of a silicon oxide film or a silicon oxynitride film.
After formation of the gate insulating film of the two-layer structure, a polysilicon film or a metal film is formed on the gate insulating film, followed by removing the excess portion of the polysilicon film or the metal film by means of a RIE etching method, thereby forming a gate electrode. It should be noted in this connection, that, after formation of the gate electrode, it is necessary to remove selectively the remaining interfacial oxide film and the high-k material film in order to expose the source region and the drain region of the silicon substrate.
In this case, it is necessary to remove selectively the interfacial oxide film (SiO2 film) and the high-k material film, for example, HfO2 film by means of the wet etching because a damage is done to the silicon substrate in the case of employing the RIE etching method. In the case of employing the wet etching method, however, serious problems are brought about as pointed out below.
A dilute hydrofluoric acid is used in general for removing the SiO2 film by the wet etching method. However, the removing rate of the high-k material film such as a HfO2 film is very low under the conditions adapted for the removal of the SiO2 film to give rise to the difficulty that a long time is required for the removal of the high-k material film. In addition, SiO2 exhibits an etching rate higher than that exhibited by the high-k material to give rise to an inconvenience that SiO2 in the element separating region is also removed in the etching stage of the high-k material film.
In order to overcome the inconvenience pointed out above, it is conceivable to use a chemical liquid exhibiting a reverse selectivity. If the two layers noted above are removed by the etching with such a chemical liquid, however, another inconvenience is generated such that an under-cut is generated in the high-k material film because the high-k material film exhibits a high etching rate.
As pointed out above, where the two layers are removed by the wet etching, an inconvenience accompanying the etching selectivity is brought about. A technology adapted for overcoming the inconvenience pointed out above is disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 2004-179583. Specifically, it is disclosed in this patent document that a wet etching is performed by using a dilute hydrofluoric acid as the chemical liquid and by defining the hydrofluoric acid concentration of the dilute hydrofluoric acid such that the two layers are etched non-selectively.
The technology disclosed in the patent document quoted above certainly permits overcoming the problems based on the etching selectivity. However, the etching conditions are not necessarily satisfactory to the two layers that are to be etched. In some cases, an inconvenience is generated such that the etching rate is rendered low.